1. Field of the Invention
The invention relates to a motor driver and a method of controlling the motor driver, and in particular to a motor driver that drives a permanent magnet motor and a method of controlling the motor driver.
2. Description of the Related Art
Typically, in a vehicle, such as an electric vehicle or a hybrid vehicle, the driving force generated by electric energy is obtained by converting the direct current voltage supplied from a high-voltage power source into a three-phase alternating-current power with the use of an inverter and rotating the three-phase alternating-current motor with the use of the alternating-current power. When the vehicle is decelerated, inversely, the regenerated energy obtained by the regenerative power generation by the three-phase alternating-current motor is stored in the power source, whereby the vehicle is driven without wasting energy.
In such a hybrid vehicle or an electric vehicle, as the three-phase alternating-current motor, a permanent magnet motor is widely used because of its high power density and high efficiency. The permanent magnet motor is controlled by controlling the amplitude and phase of the electric current that flows through the permanent magnet motor by switching the switching devices constituting the inverter at a high frequency equal to or higher than a few kilohertz.
For example, in Japanese Patent Application Publication No. H9-275696 (JP-A-9-275696), a controller for controlling drive of a permanent magnet motor is disclosed that has a high-frequency electric current supply means for supplying high-frequency electric current to the coils of the stator when the temperature of the permanent magnet is equal to or below a predetermined value. According to this document, because there is a possibility of the occurrence of demagnetization due to the reduction in coercivity of the permanent magnet in a low temperature environment, under low temperature conditions, high-frequency magnetic field is generated by the high-frequency electric current that is supplied to the coils of the stator, whereby the permanent magnet is heated by the iron loss caused in the core portion as the high-frequency magnetic field is formed.
In the inverter that drives the permanent magnet motor, when switching devices are switched on and off, the ripple current depending on the switching frequency is generated. The ripple current increases as the switching frequency is reduced. Thus, it is preferable that the switching frequency be reduced to suppress the electric power loss caused in the inverter during switching operation. However, when the switching frequency is reduced, the ripple current increases.
When the ripple current increases, in the permanent magnet motor, harmonic loss is caused because the magnetic flux density therein includes harmonic components. In particular, when a rare-earth permanent magnet is used, the electrical conductivity is relatively high and therefore, there is a possibility that eddy currents are generated in the permanent magnet and increase the loss. Such increase in the eddy-current loss increases the magnet temperature and thus causes demagnetization in the permanent magnet motor. Due to the occurrence of such demagnetization, a problem can arise that the operational efficiency (rotation efficiency and power generation efficiency) is reduced in the permanent magnet motor. However, in the above-cited JP-A-9-275696, an object is to prevent the occurrence of demagnetization caused in the permanent magnet motor under low temperature conditions and there is no mention concerning the means for preventing the occurrence of demagnetization due to such increase in the magnet temperature.